English: At the given coordinates you stand surrounded by a circle of impressive stones. These five stone groups are part of an art work and results of the project "Global Stone". Together with their siblings on the five continents they symbolize the five steps on the path to peace. More details about this project can be found on the official project page and Wikipedia.

One of the groups of stones is an outstanding example of orbicular granite, a rather rare kind of granite, that contains spheroidal structures, as shown in the images above. The origin of these structures is not clarified completely, probably they are created in a cooling magma by crystallization. The differences in type and the structure of the spheres can be explained by a multi-stage crystallization process.

Orbicular rocks are rare textural varieties of magmatic (plutonic) rocks. They occur in all types of composition from granitic to mafic, ultramafic and carbonatic rocks. Most of them are composed of the same minerals as lots of plutonic rocks: K-feldspar and plagioclase, quartz, dark minerals like hornblende or biotite and some other minerals. The typical features of orbicular rocks are round bodies, called "orbicules" or sometimes just "orbs". These orbicules show a wide variation in size and composition. Some of them are elongated due to deformation.

All of them contain a core in the middle surrounded by radially grown crystals. The outer part of an orbicule shows concentric lines - called "shells". Some rocks have only got one single shell, but most of the rocks contain a lot of concentric shells.

As said above – orbicular rocks are rare. One reason for that is the tiny size of their occurences. Nearly all orbicular rocks are located at small dykes and lenses at the margin of magmatic intrusions. The average size of such an occurence is about some ten meters (!) in length and 3-5 meters in width. Bigger outcrops may be found but they are extremely rare. On the other hand a couple of very little occurrences is known and some of them are smaller than 5 meters in diameter.

The Orbicular Granite placed here provides a geological oddity and comes from one of the oldest known deposit near Mount Magnet, which has an age of more than 2 billion years. Due to the polishing and their position here in the Tiergarten the stones show the beauty of Orbicular Granite in an impressive way.

To log this earth cache you must visit the five stone groups in the Tiergarten, identify the stone group containing the Orbicular Granite and answer the following questions (Magnifier and measuring tape are recommended here):

How would you describe the form of the orbicules typical for the deposit near Mount Magnet?

How much do you estimate the average diameter of the orbicules?

How would you describe the edges of the orbicules? Clearly separated from the surrounding matrix or rather interwoven with her?

Is the matrix rather homogeneous or can you find smaller crystals embedded into it?

From which continent the Orbicular Granite shown here in the Tiergarten comes from?

Please send me the answers to my GC account. You can log the cache immediately, I will contact you if something is wrong. Logs without a previous e-mail will be deleted.

Request of the owner: a photo of you or your GPS in front of the stones would be great! (Spoiler pictures are ok, too.)

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